1. Field of the Invention
This invention relates to an optical pickup device for writing information signals onto and reading information signals from an optical recording medium and also to an optical disc device provided with such an optical pickup device and adapted to record and reproduce information signals, using an optical disc as optical recording medium.
2. Related Background Art
Optical discs are known as optical recording medium and various optical pickup devices for reading information signals from an optical recording medium have been proposed to date.
Popular optical discs include “Compact Discs” (tradename, to be referred to as “CD” hereinafter) and “Digital Versatile Discs (tradename, to be referred to as “DVD” hereinafter) that can store information signals much more densely than CDs. Meanwhile, optical pickup devices adapted to read information signals from both “CDs” and “DVDs” are also known.
Referring to FIG. 1 of the accompanying drawings, an optical pickup device adapted to read information signals from both “CDs” and “DVDs” comprises a laser diode (LD) 101 operating as light source. The laser beam emitted from the laser diode 101 is typically red light (e.g., having a wavelength of 635 nm) and fed to a beam splitter 103 by way of a diffraction grating 102. The diffraction grating 102 is used to generate a sub-beam for detecting a tracking error signal. The beam splitter 103 is a plate having a pair of parallel surface planes that are inclined by 45 relative to the optical axis of the laser beam coming from the laser diode 101. The laser beam emitted from the laser diode 101 is reflected and deflected by 90 by the corresponding surface plane of the beam splitter 103 before it is collimated by a collimator lens 104 and enters an objective lens 105. The objective lens 105 focuses the incident laser beam on the signal recording surface of a “DVD” 106a or a “CD” 106b. 
The laser beam focused on the signal recording surface of either the “DVD” 106a or the “CD” 106b is modulated according to the information signal recorded on the “DVD” 106a of the “CD” 106b, whichever appropriate” and reflected so that it returns to the objective lens 105 as reflected laser beam. The reflected laser beam then gets to the beam splitter 103 by way of the collimator lens 104. As the reflected laser beam is transmitted through the beam splitter 103, it gives rise to astigmatism and is subsequently focused on the light receiving surface of a photodetector (PD) 107. A focusing error signal, if any, can be detected on the basis of the astigmatism generated as a result of being transmitted through the beam splitter 103.
If the laser beam emitted from the light source is red light having a single wavelength, it cannot read any information signal from a “CD-R” that uses a coloring matter for the signal recording layer. This is because the reflectivity of the signal recording surface of a “CD-R” is very low relative to red light.
In view of this fact, there has been proposed an optical pickup device comprising a pair of light sources that are adapted to emit beams with different wavelengths as shown in FIG. 2 so that it may read information signals not only from “CDs” and “DVDs” but also from “CD-Rs” whose operation is highly dependent on the wavelength of light to be used with it.
With such an optical pickup device, a beam of red light (e.g., having a wavelength of 635 nm) is emitted from a laser diode 101 operating as the first light source and an infrared beam (e.g., having a wavelength of 780 nm) is emitted from a laser chip comprising a light receiving/light emitting composite element 109 and operating as the second light source.
The laser beam emitted from the laser diode 101 is fed to a beam splitter 103. The beam splitter 103 is a plate having a pair of parallel surface planes that are inclined by 45 relative to the optical axis of the laser beam coming from the laser diode 101. The laser beam emitted from the laser diode 101 is reflected and deflected by 90 by the corresponding surface plane of the beam splitter 103 before it is collimated by a dichroic beam splitter 108 and a collimator lens 104, and enters an objective lens 105. The objective lens 105 focuses the incident laser beam on the signal recording surface of a “DVD” 106a. 
On the other hand, the laser beam emitted from the laser chip comprising the light receiving/light emitting composite element 109 is fed to a dichroic beam splitter 108. The dichroic beam splitter 108 has a reflection plane that is inclined by 45 relative to the optical axis of the laser beam coming from the laser chip comprising the light receiving/light emitting composite element 109. The laser beam emitted from the laser chip is reflected and deflected by 90 by the reflection plane. The laser beam emitted from the laser chip comprising the light receiving/light emitting composite element 109 and the laser beam emitted from the laser diode 101 are made to have a same and identical optical axis. The laser beam emitted from the laser chip comprising the light receiving/light emitting composite element 109 is then collimated by the collimator lens 104 and enters the objective lens 105. The objective lens 105 focuses the incident laser beam on the signal recording surface of a “CD” 106b. 
The laser beam focused on the signal recording surface of either the “DVD” 106a or the “CD” 106b is then reflected by the signal recording surface thereof so that it returns to the objective lens 105 as reflected laser beam. The reflected laser beam then gets to the collimator lens 104 and the dichroic beam splitter 108. Since the dichroic beam splitter 108 transmits red light but reflects infrared beams, the optical path of the red beam and that of the infrared laser beam are separated from each other there.
The red beam transmitted through the dichroic beam splitter 108 then gets to the beam splitter 103 and, as it is transmitted through the beam splitter 103, it gives rise to astigmatism and is subsequently focused on the light receiving surface of a photodetector (PD) 107.
On the other hand, the infrared beam reflected by the reflection plane of the dichroic beam splitter 108 is focused on the light receiving surface of the photodetector of the light receiving/light emitting composite element 109.
Meanwhile, as a result of the advancement of semiconductor technologies in recent years, it has become possible to mount a pair of laser chips on a same semiconductor substrate as shown in FIG. 3 by using the so-called monolithic technology. More specifically, the light emitting spots 111a, 111b of a pair of laser chips can be arranged transversally side by side with a gap of only 80 m to 200 m separating them.
A light receiving/light emitting composite element 110 comprising a pair of laser chips 111a, 111b can be formed by arranging a photodetector 112 on a semiconductor substrate 114 in addition to the laser chips 111a, 111b. The light receiving/light emitting composite element 110 is additionally provided with a prism 113 arranged on the photodetector 113 and having its sloped plane faced to the laser chips 111a, 111b. 
With the light receiving/light emitting composite element 110, the laser beams emitted from the laser chips 111a, 111b are reflected by the sloped plane of the prism 113 and directed to the outside of the light receiving/light emitting composite element 110. Then, they are reflected back to the light receiving/light emitting composite element 110 by the corresponding optical recording medium to enter the prism 113 and become detected by the photodetector.
Referring now to FIG. 4, with the optical pickup device that is adapted to read information signals from both a “CD” and a “DVD” by using a monolithic laser diode, the laser beams emitted from the laser chips 111a, 111b of the light receiving/light emitting composite element 110 are fed to the objective lens 105 by way of the collimator lens 104 so as to be focused on the signal recording surface of the “DVD” 106a or the “CD” 106b by the objective lens 105. Then, the laser beam reflected by the signal recording surface of the “DVD” 106a or the “CD” 106b, whichever appropriate, is fed back to the light receiving/light emitting composite element 110 and received by the photodetector of the light receiving/light emitting composite element 110.
In the light receiving/light emitting composite element 110, the pair of laser chips 111a, 111b that are used respectively for a “DVD” and a “CD” are separated from each other by a gap of about 120 m. Then, the two laser beams emitted from the respective laser chips 111a, 111b are made to strike the optical recording medium, keeping the distance of 120 m separating their optical axes from each other, so as to be reflected by the signal recording surface of the optical recording medium and fed back to the photodetector 112 of the light receiving/light emitting composite element 110.
The photodetector 112 has a first light receiving surface for receiving the laser beam to be used for a “DVD” that is reflected by the signal recording surface of a “DVD” and a second light receiving surface for receiving the laser beam to be used for a “CD” that is reflected by the signal recording surface of a “CD”. The first and second light receiving surfaces are separated by a gap of about 120 m, which is same as the gap separating the laser chips 111a, 111b. 
The above described optical system is so regulated that the laser beam to be used for a “DVD” occupies the center of the optical axis of the optical system and hence the laser beam to be used for a “CD” is displaced from the optical axis by the distance same as the distance separating the light emitting spots of the laser diode, or 120 m, in view of the fact that the operation of reading information signals from a “DVD” is more difficult than that of reading information signals from a “CD”. The use of a hologram element has been proposed to correct the displacement of the laser beam due to the displaced light emitting spot thereof.
The use of a monolithic laser diode for an optical pickup device provides advantages including a reduced number of components, down-sizing and easier regulating operations during the manufacturing process.
It is also possible to form an optical system, using a monolithic diode in a discrete way as shown in FIG. 5. The laser diode 101a of the optical pickup device of FIG. 5 comprises first and second laser chips 111a, 111b as shown in FIG. 6. The laser beams emitted from the laser diode 101a typically include a red laser beam and an infrared laser beam that are fed to a beam splitter 103 by way of a diffraction grating 102. The diffraction grating 102 is used to generate a sub-beam for detecting a tracking error signal. The beam splitter 103 is a plate having a pair of parallel surface planes that are inclined by 45 relative to the optical axis of the laser beam coming from the laser diode 101a. The laser beam emitted from the laser diode 101 a is reflected and deflected by 90 by the corresponding surface plane of the beam splitter 103 before it is collimated by a collimator lens 104 and enters an objective lens 105. The objective lens 105 focuses the incident laser beam on the signal recording surface of a “DVD” 106a or a “CD” 106b. 
The laser beam focused on the signal recording surface of either the “DVD” 106a or the “CD” 106b is modulated according to the information signal recorded on the “DVD” 106a or the “CD” 106b, whichever appropriate and reflected so that it returns to the objective lens 105 as reflected laser beam. The reflected laser beam then gets to the beam splitter 103 by way of the collimator lens 104. As the reflected laser beam is transmitted through the beam splitter 103, it gives rise to astigmatism and is subsequently focused on the light receiving surface of a photodetector (PD) 107. A focusing error signal, if any, can be detected on the basis of the astigmatism generated as a result of being transmitted through the beam splitter 103.
With this optical pickup device again, the light receiving section 107a to be used for a “DVD” and the light receiving section 107b to be used for a “CD” of the photodetector 107 are separated from each other by a distance same as the gap separating the light emitting spots of the two laser chips 111a, 111b as seen from FIG. 7.
A discrete optical system is advantageous relative to an integrated optical system as shown in FIGS. 3 and 4 because it involves less diffracted light that is unnecessary to the system and it can be manufactured more easily.
With any of the above described optical pickup devices comprising a pair of light emitting spots, the two light emitting spots are separated from each other at least by a distance of about 80 m in view of the spatial restrictions imposed on it. Thus, in the case of a confocal optical system, two focal points are formed on the respective light receiving surfaces of the photodetector and separated from each other by a distance of about 80 m. Therefore, a pair of light receiving surfaces are arranged in the photodetector and separated from each other by at least about 80 m in order to receive the two laser beams that are focused to the respective focal points.
If the optical disc to be used with such an optical pickup device is driven to rotate at high speed in order to read information signals therefrom as in the case of a “CD-ROM”, it is necessary to arrange a pair of I-V amplifiers near the respective light receiving surfaces on the semiconductor substrate of the photodetector. However, with the photodetector of any of the above described optical pickup devices, it is highly difficult to arrange such a pair of I-V amplifiers on the semiconductor substrate because the light receiving surfaces are arranged so tightly relative to each other with such a narrow gap separating them.
This problem may be dissolved by separating the two light emitting spots by a large gap. However, with a pair of light emitting spots that are separated from each other by a large distance, one of the laser beams will inevitably be displaced from the optical axis of the optical system also by a large distance to consequently degrade the optical performance of the optical system.